Deciphering interferon functions in avian influenza using receptor knockout models in the natural host.

Why do some poultry flocks survive an influenza outbreak while others succumb to devastating, rapid mortality? Understanding the avian immune system is the frontline defense against highly pathogenic avian influenza, a virus that poses a massive zoonotic threat to humans. Scientists have long known that interferons (IFNs)—cytokines (signaling proteins) that act as the body's "alarm system"—are central to this defense.

However, the specific roles of the two main players, Type I (IFN-α/β) and Type III (IFN-λ) interferons, have remained blurred. In mammals, these systems often overlap. This makes it difficult to tell which one stops a virus and which one prevents the immune system from attacking the host itself. To solve this, researchers have engineered specialized chicken models that lack these specific receptors. This allows them to study one system without affecting the other.

The limits of mammalian proxies

Current strategies for controlling avian influenza rely heavily on biosecurity and mass depopulation. These methods carry enormous economic costs. While much is known about how humans and mice respond to influenza, relying on mammalian models is risky. The innate immune mechanisms (the body's immediate, non-specific defense) in chickens can differ fundamentally from those in mammals. A drug that works in a mouse might fail in a hen.

Furthermore, the signaling pathways of Type I and Type III IFNs are notoriously difficult to decouple. They induce many of the same interferon-stimulated genes (ISGs)—proteins that act like cellular security guards to block viral replication. Traditional studies often see the combined effect of both. Without a way to selectively silence one, researchers cannot determine if an immune response is a targeted defense or a byproduct of generalized inflammation.

Engineering precision via PGCs

To bypass this ambiguity, the authors utilized chicken primordial germ cells (PGCs)—the precursor cells that eventually form sperm and eggs. They used these to create stable, heritable knockout (KO) lines. This approach is akin to rewriting the source code of a computer program. It ensures a specific function can no longer be called, regardless of how many times the program runs.

The researchers followed a precise engineering workflow: 1. They designed guide RNAs to target the coding regions of the IFNAR1 (Type I receptor) and IFNLR1 (Type III receptor) genes. 2. Using CRISPR/Cas9, they co-transfected these guides into PGCs. They used a Cas9-2A-eGFP vector. The eGFP acts as a visual tag. It allows researchers to identify successfully edited cells via fluorescence. 3. For the Type I KO, they used homology-directed repair (HDR) to introduce specific deletions. These create premature stop codons. This effectively breaks protein production. 4. These edited PGCs were then injected into embryos to create germline chimeras. These were bred to establish homozygous lines that lack the receptors entirely.

As shown in, this method successfully produced chickens that are functionally deficient in either Type I or Type III signaling.

These birds showed normal growth and fertility.

Divergent roles in viral defense

The study reveals that these two interferon systems have distinct specializations. The authors report that Type I IFN is the primary driver of early, systemic defense. In experiments involving the H3N1 influenza strain, the paper finds that IFNAR1-/- hens suffered from a rapid onset of severe symptoms. They also had significantly higher viral loads in the trachea and cloaca compared to wild-type birds [Figure 7c, d].

In contrast, the role of Type III IFN appears more localized and regulatory. The researchers observe that while Type III IFN contributes to the initial antiviral state, its absence leads to different pathological outcomes. Specifically, the authors report that in the oviduct—a key site for H3N1 infection—Type III IFN signaling promotes inflammation. In WT hens, H3N1 caused moderate inflammation (Magnum grade 2). However, IFNLR1-/- hens showed milder inflammation (grade 1) [Figure 7g, h].

Perhaps most strikingly, the authors find that the loss of Type I signaling triggers a breakdown in the body's natural braking mechanisms. In IFNAR1-/- hens, the lack of an effective antiviral state prevents the activation of negative feedback loops. These loops involve the SOCS1 and SHP2 proteins. The paper reports that this leads to an uncontrolled increase in IFN-α/β secretion. Levels were approximately 10 times higher than in WT birds [Figure 7f]. This could potentially drive a "cytokine storm" (an overactive immune response).

Complexity in the viral landscape

While the results are compelling, the paper does not claim a universal rule for all viruses. The effectiveness of these interferon pathways is highly strain-specific. For example, in embryonic studies, the researchers report that both Type I and Type III IFNs were necessary to restrict the WSN33 (H1N1) strain. However, the response to H9N2 appeared to rely more heavily on Type I signaling [Figure 6a].

There are also notable trade-offs in the biological data. The authors note that while Type III IFN can be pro-inflammatory in certain contexts, such as the H3N1-infected oviduct, it may have beneficial anti-inflammatory roles in other scenarios. This complexity means that any therapeutic attempt to boost one type of interferon must account for the specific viral strain and the tissue being targeted. Finally, the study focuses on specific receptor knockouts. It does not explore how the loss of multiple different cytokine families might interact with these existing deficiencies.

A new toolkit for avian immunology

The verdict is a clear "yes" for the utility of these models. The authors have successfully moved beyond the limitations of mammalian proxies. They have provided a high-fidelity, genetically defined toolkit tailored to the natural host of avian influenza. These knockout lines allow researchers to move from observing correlations to testing specific causal mechanisms in the chicken immune system.

For practitioners looking to develop vaccines or antiviral therapies, the takeaway is nuanced. Type I IFN is the hammer used for early viral suppression. Meanwhile, Type III IFN acts more like a thermostat. It regulates the inflammatory response in mucosal (lining) tissues. Future research using these models will likely be essential for designing "smart" immunomodulators. These could suppress viral replication without triggering the catastrophic inflammation seen in the Type I-deficient models.

Figures from the paper